The necessity for reducing dependency on petroleum, increasingly more scarce and expensive, allied to growing worldwide concern to stop the rise in the temperature of the planet, is leading humanity to take a series of steps in terms of utilising other types of fuels which permit controlling the emission of gases causing the greenhouse effect, without allowing this to negatively affect the development of our industry.
Burning fossil fuels is one of the principal sources of generation of CO2, considered to be most responsible for aggravation of the greenhouse effect. To control such generation, one of the principal mitigating actions adopted worldwide has been the alternative burning of fuels originating from renewable sources. Furthermore, it is also of great importance to reduce the consumption of petroleum and, having this objective, to increase the offer of synthetic and semisynthetic fuels.
The worldwide trend in terms of the utilisation of fuels not derived from petroleum, renewable or not, commenced with the land transport mode and has now reached with considerable strength the aeronautical industry.
According to the International Air Transport Association (IATA) world aviation is responsible for 2% of the total volume of emissions of carbon dioxide produced by man.
IATA, in its annual report (‘2009 Report on Alternative Fuels’), announced that the associates thereof assume the obligation by 2017 of utilising 10% alternative fuels in the inventory thereof of aviation fuels consumed annually.
The legislation of the European Union has furthermore just brought aviation within the EU Emissions Trading Scheme (ETS) by means of Directive EC/2008/101, with a view to preparing it to satisfy the requirements of the directive.
The European Commission (EC) has drawn up a paper entitled ‘Guidelines for the monitoring and reporting of aviation activity data’ guiding those airlines falling within the scope of the directive to take, as from 1 Jan. 2010, a series of steps in terms of monitoring emissions of CO2 and controlling consumption by aircraft operating in or overflying Europe.
IATA classifies the diverse families of fuels not derived from petroleum possessing properties similar to standard aviation kerosene (QAV) as alternative aviation fuels (Alternative Jet Fuels (AJFs)).
AJFs may be derived from biological materials as well as from coal and natural gas. The majority of AJFs may be categorised as synthetic paraffinic kerosenes (SPKs); should such SPKs be derived from biomasses other than coal or natural gas they are categorised as bioSPKs.
Synthetic hydrocarbons are synthetic fuels not restricted to the kerosene band and may be produced from coal liquefaction (Coal-to-Liquid (CTL)), from gas liquefaction (Gas-to-Liquid (GTL)), or by biomass liquefaction (Biomass-to-Liquid (BTL)) by means of the Fischer-Tropsch (FT) process. They are generically denominated XTLs or fuels produced by liquefaction of any of the aforesaid raw materials by the FT process and are considered to be variants of SPKs.
Instead of being subjected to the FT process a natural oil may be pyrolysed (thermal cracking of coal, natural gas or biomass), the oxygens thereof being removed generating a liquid fuel having properties similar to those of kerosene. These products are called hydrodeoxygenated oils (HDOs) and by virtue of the fact that they are not paraffinic they are not considered to be SPK variants.
A further common biofuel production practice is the utilisation of vegetable oils extracted from seeds, such as soya and canola. These vegetable oils are thermally cracked and subsequently chemically improved through the addition of hydrogen by means of a hydrotreatment process. The kerosene fraction of these products is called renewable hydrotreated aviation kerosene (Hydrotreated Renewable Jet (HRJ) Fuel), another variety of SPK.
Vegetable oils hydrotreated simply for the production of diesel, not usually achieving the quality necessary for the production of aviation kerosene, are called hydrotreated vegetable oils (HVOs).
There is a further category of AJFs not included in the current IATA classification, possibly because to date it has only been used to produce biodiesel and not biokerosene (bioSPK), being the category of alkyl esters of fatty acids.
The process for the production of alkyl esters of fatty acids through catalysed transesterification of oils of natural origin, already in the public domain in so far as the production of biodiesel is concerned, still requires improvement to be suitable for the production of finished aviation biokerosene.
Vacuum fractionation of hydrocarbons is a refining process utilised when it is desired to obtain lighter distilled products with minimum thermal cracking of the residual heavy fractions. It should be mentioned that none of the alternative aviation fuels cited herein has received certification from aircraft manufacturers to be used pure. All the products have been used in mixtures with conventional aviation kerosene, it being recorded that the maximum quantity utilised was a concentration of 50% by volume of a synthetic paraffinic kerosene with a conventional aviation kerosene.
That is to say, all those utilised to date have been semisynthetic aviation kerosenes and such utilisation under actual flight conditions has only become possible following the creation of specification ASTM D7566 ‘Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons’, now adopted worldwide in the certification of these types of product.